1. Field of the Invention
This invention generally relates to the field of electrical safety compliance testing, and more particularly to a multiple function safety compliance test instrument that includes:                a high voltage output switching relay for selectively supplying power from either a line voltage source or a high voltage source to a device under test (DUT); and        a circuit breaker for protecting the high voltage output switching relay from high currents.        
The invention replaces the conventional circuit breaking relay or fuse with a protective circuit that includes transistors connected in series between the line voltage source and the switching relay, the transistors having a relatively small ON resistance to enable the transistors to carry currents during the entire test time and not just at start-up. The short turn-off time of the series-connected transistors reduces the current-carrying requirements of the high voltage output switching relay, enabling the use of a medium rather than high current relay to handle switching between the line voltage source and the high voltage source.
2. Description of Related Art
Electrical safety compliance test instruments are used by manufacturers to test electrical products for hazards such as insulation or ground faults that could cause injuries to purchasers and users of the products. Conventional safety compliance tests include continuity, ground bond, AC and DC dielectric withstand or “hipot,” insulation resistance, line leakage, and run tests. For convenience, it is common for safety compliance instruments to be capable of performing more than one of these tests.
Descriptions of various types of safety compliance tests and multi-function safety compliance test instruments may be found in commonly-assigned U.S. Pat. Nos. 6,011,398; 6,054,865; 6,515,484; 6,538,420; and 6,744,259, incorporated by reference herein. Each of the multiple function safety compliance test instruments disclosed in these patents includes a high voltage source, a low voltage power source, and a switching matrix for selectively supplying high or low voltages from the respective sources to a DUT via output ports. One of the output ports in the switching matrix serves as a neutral or ground, while the other port is connected to a high voltage switching relay that is, in turn, connected to the high voltage source and to a source of low voltage, such as a power main or line voltage source.
Line voltage tests typically involve higher currents (277V/15 A) than high voltage tests (5KV/40 mA). Short circuits that occur in the DUT during line voltage tests, or that occur during changing of a test set-up, can draw sufficient current to damage the high voltage switching relay. As a result, it is conventional to connect a fuse or circuit breaker between the line voltage source and the switching relay. The circuit breaker conventionally takes the form of a mechanical line voltage source controlling relay responsive to a current sensor. Transistors are used to momentarily turn on the power supply to the DUT, but then are bypassed by the mechanical relay to eliminate the heating and voltage drops that occur in the transistors.
Because of the relatively slow reaction time of relays (or fuses), which permits currents to exceed or overshoot a shut-off threshold before shut-off can actually occur, a typical high voltage switching relay protected by a conventional fuse or relay must be capable of handling both high voltages from the high voltage source and high currents from the line voltage source. The line voltage source controlling relay and the switching relay are both relatively expensive components and contribute significantly to the overall cost and size of the safety compliance test instrument.
The present invention replaces the relay or fuse with solid state circuitry. Examples of known solid state or electronic circuit breaker circuitry are disclosed in U.S. Pat. Nos. 4,148,088; 4,853,820; and 4,979,068; and U.S. Patent Publication Nos. 2002/0080544 and 2003/0202304. However, none of the conventional solid state circuit breaker arrangements is suitable for use in a safety compliance test instrument, which by definition must be capable of handling currents and voltages in excess of those normally handled to the products being tested. Because line voltage tests require a relatively high current even during a normal run, the ON resistance of a transistor used in a conventional electronic circuit breaker is sufficient to cause a significant voltage drop, and consequent heating, precluding use of the transistor as an in-line circuit breaker without the addition of a large heat sink. As a result, circuit-breaking relays rather than electronic circuit breakers continue to be used in all commercially-available safety compliance test instruments.